Add SkSurface::asyncReadPixels()

Initial version. Current limitations: No Metal support, no color space
conversions, for each src color type only one dst color type is legal (
which may or may not be the src color type), no alpha type conversions.

Bug: skia:8962

Change-Id: I6f046a32342b8f5ffb1799d67d7ba15c250ef9bf
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/212981
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
20 files changed